Preparation of carboxyalkyl ethers of cellulose



Patenied Aug. 8, 1950 PREPARATION OF CARBOXYALKYL ETHERS OF CELLULOSEEugene D. King and John S. Tinsley, Wilmington, Del., assignors toHercules Powder Company, Wilmington, Del., a corporation of Delaware NoDrawing. Application December 10, 1946, Serial No. 715,340

9 Claims.

This invention relates to a process for the preparation of carboxyalkylethers of cellulose and. more particularly, to an improvement in themedium of reaction for such preparation. Carboxyalkyl ethers, such ascarboxymethylcellulose, are in general prepared by reacting cellulosewith a carboxyalkyl etherifying agent, usually a halogenated lower fattyacid or salt thereof, in the presence of free alkali and water. Wherethe reaction proceeds to the formation of a product of substitution inexcess of 0.4! hydroxyl group per anhydroglucose unit of cellulose, theinitially heterogeneous mixture becomes a heavy dough because of thesolubility of the carboxyalkyl ethers in the aqueous alkali present.

The dough formed in the ordinary reaction is dimcult to agitate and isdimcult to process at the end of the reaction. The reaction productcontains occluded salt and other impurities. It is necessary toprecipitate the product from the 2 hour at a temperature varying from 28to 32 C. during the addition. Agitation was continued for one-half hour,during which time the temperature of the reaction mass was graduallyreduced to 17 C. Seventy-three parts of solid monochloracetic acid wasthen added. over a dough and then to treat the precipitate extensivelyto remove these impurities.

Now, in accordance with this invention, the difliculties hereinabovementioned are overcome by reacting cellulose in the presence of freealkali with a carboxyalkylating agent in a medium comprising suficientisopropyl alcohol or tertbutyl alcohol to maintain the carboxyalkylcellulose formed, in the solid, suspended state throughout the entirereaction. The desired product is then readily recovered merely bydraining oil or centrifuging ofl the liquid medium and washing the solidproduct to remove impurities. The difficult precipitation stepnecessitated in prior art processes is not required nor used since theproduct is in substantially the same physical state as the celluloseused as a starting material. Thus, in the ordinary case, where fibrouscellulose is used, the product will likewise be fibrous. Such a productis very easy to purify and is in a highly desirable form for use.

The invention will be illustrated by typical examples of the preparationof carboxymethylcellulose, which is representative of cellulosecarboxyalkyl ethers generally. All parts are by weight unless otherwisespecified.

EXAMPLE 1 Sheets of. chemically purified cotton linters of a mediumviscosity type were shredded. Fifty parts of this shredded cellulose wasthen suspended in 789 parts of tert-butyl alcohol. The resulting slurrywas vigorously stirred while 208.3 parts of a 30% aqueous solution ofsodium hydroxide was added over a period of one-half period of one-halfhour at a temperature rising to 21 C. during this time. The reactionmixture so obtained was then heated to 55 C. in one and one-halfhoursand kept at 55 C. for four additional hours, agitation being continuedthroughout the reaction. The reaction product at this point was fibrousand similar in appearance to the original shredded sheet linters. Thereaction medium was then drained from the fibers. The fibrous productwas then stirred in 70% methanol (30% water) and neutralized, while insuspension, with the acetic acid. The neutralized product was thendrained and washed with additional 70% methanol, dehydrated withanhydrous methanol, and dried in air at 70 C. The final product had asubstitution of 0.88 and was readily soluble in water. (Substitutionherein refers to the average number of hydroxyls 25 present peranhydroglucose unit of cellulose which have been substituted in thereaction, a substitution of 3 representing complete substitution of thecellulose.)

EXAMPLES 2-4 In these examples, isopropyl alcohol was utilized as thenonsolvent reaction medium. The general procedure was similar to that ofExample 1, but the concentration of the aqueous sodium hydroxide addedto the suspension of cellulose in isopropyl alcohol was varied toillustrate how products of various substitution are readily obtained asdesired by the process according to the present invention. In each case,30 parts of shredded sheet purified linters was suspended in theindicated parts of isopropyl alcohol. One hundred per cent isopropylalcohol was used in Example 2 and 87% isopropyl alcohol in Examples 3and 4. Thirty parts by weight (on the dry basis) of sodium hydroxide wasthen added in each case in the form of an aqueous solution of theconcentration indicated in Table I, the percentage figures givenrepresenting the concentration of the sodium hydroxide before theaddition of the monochloracetic acid and taking into account the waterwhich was present in the isopropyl alcohol. This addition was made overa period of one-half hour with agitation. Thirtyfive parts ofchloracetic acid was then added over an additional period of one-halfhour. The reac- 3. tion in each case was completed and the productrecovered in the manner of Example 1. The products varied insubstitution as given in Table I.

Thirty parts of shredded sheet purified cotton 15 linters was suspendedin 526 parts of 87% isopropyl alcohol plus 196 parts of anhydrousisopropyl alcohol. The resulting slurry was agitated and 20 parts of anaqueous solution of sodium hydroxide in water was added. Theconcentration of the sodium hydroxide solution was 20%. Two parts ofchloracetic acid dissolved in 8 parts of isopropyl alcohol was thenadded over a one-halt hour period. The resulting mixture was heated to55 C. in one hour and held at that temperature with agitation for fivehours. The resulting fibrous product was then separated from the liquidmedium by centrifuging, washed with methanol, and dried at 100 C. inair. It had a substitution 01. 0.11. It was insoluble in water butsoluble in aqueous sodium hydroxide solution at 0-5 C.

EXAMPLEG This example gives the results of tests using various alcoholsas reaction media and for purpose of comparison gives the results oftests using isopropyl alcohol under the same conditions. Procedures forcarrying out the tests were generally similar to those hereinbetoredescribed. The comparative results are shown in Table II below.

tertiary butyl alcohol are unique in the process of this invention. Thereasons for the successtul results obtained with these alcohols are notclear, but it is believed that they combine substantial inertness towardthe reactants with ability to take up water from the other reactantswhile maintaining a condition oi nonsolvency tor the product.Nonsolvency alone, however, i: insuflicient since it has been found thatdiluents such as hexane, which do not dissolve the product, do notprevent formation of a dough oi the cellulose ether in the aqueousalkali present.

The isopropyl or tertiary butyl alcohol will be utlized in a quantitysuflicient to prevent the product from going into solution in thereaction medium. This represents a minimum quantity. As a rule, somewhatmore will be used to provide for free agitation of the mixture. Anunlimited quantity can be used, subject only to the inconvenience ofunnecessary dilution. In general, satisfactory ratios vary from fiveparts to parts of the alcohol for each part of cellulose. A quantity ofabout 15 to 30 parts 01' alcoh.l per part oi cellulose is preferred. F.

The alcohol may be in the anhydrous form as added or it may contain acertain amounto! water; i. e., it may be in the form of the constantboiling mixture in which it is obtained on separation from water. Itwill be understood, of course, that if water is added in the alcohol,due allowance must be made therefor so that the reaction medium has thedesired total water content. As a rule, the ratio of alcohol to water inthe reaction system is greater than about 1.5 parts of alcohol per partof water.

The present invention is concerned with the preparation of anycarboxyalkyl cellulose insoluble in the alcohols used as the reactionmedium. The examples illustrate preparation 01' carboxymethylcellulose.Other carboxyalkyl ethers, such a carboxyethylcellulose,carboxypropyicellulose, carboxybutylcellulose, and sub- Table II arseSubsti- Reaction Medium tution 8 1 M00 0] Water I Solubility Named vesUnder the Same Conditions Math 1 Alcohol 0.16 0. 0 Insoluble. EthyAlcohol. 0. 35 0. 9 Do. n-Propyl Alcohol- 0.38 0.65 to 0. 70 Few Fibers,Very Granular. n-Buty] AlcohoL. 0.61 0. 9 Very Few Fibers, QuiteGranular. Iso-Butyl Alcohol- 0. 31 0. to 0. Opaque Gel. 8ec-ButylAlcohol" 0. 37 0. 65 to 0. 70 Do. Tert.-Butyl Alcohol- 0.88 0. 9Excellent. Acetone 0. 39 0. 65 to 0. 70 Almost Fiber Free, QuiteGranular. Dioxane. 0. 45 0. 65 to 0. 70 Do.

1 Two per cent aqueous solutions.

Example 6 shows the marked superiority both 60 stituted variations inthe alkyl group of the in efliciency and in maximum extent ofsubstitution, obtained by the use of isopropyl alcohol and tertiarybutyl alcohol as compared with the other alcohols tested. With theexception of the prodnot obtained with tertiary butyl alcohol, thesolution quality in all cases is definitely interior to that which isobtained from products prepared with isopropyl alcohol under the samereaction conditions. Substitution values are from about 0.2 to 0.4 lessthan with isopropyl alcohol and tertiary butyl alcohol, if the unusuallybad methyl alcohol is excluded. These results are indicative of adifierence in behavior of the two alcohols mentioned and the otheralcohols tested.

ethers mentioned, are, for example, prepared in a similar fashion. Itwill be appreciated that the primary reaction product is usually thealkalimetal salt of the carboxyalkyl cellulose, since the reaction iscarried out in a strongly alkaline medium. The free acid is, in eachcase, readily obtained by treating the primary product with a mineralacid. Other salts are readily prepared from the free acid.

10 The etheriflcation agent illustrated in the examples is chloraceticacid, or more accurately sodium chloracetate, since the chloracetic acidis neutralized by free alkali upon addition to the reaction mixture. Itmay be added as the free It has been found that isopropyl alcohol and 1!acid or a a salt as desired, due allowance being made for theconsumption or lack of consumption of alkali already present. Instead ofchloracetic acid, other halogenated lower fatty acids, such aschlorpropionic acid, chlorbutyric acid, and salts thereof, with sodium,potassium, and the like, may be utilized. The corresponding bromine andiodine compounds are suitable but more expensive. The quantity ofetherifying agent will be varied with the substitution desired in theproduct. In general, the etheriiying agent will be used in a quantitybetween about 0.01 part and about 3.0 parts per part of cellulose. Theproportion of etheriiying agent to free alkali present may also be afactor in determining substitution. In most cases, it has been foundpreferable to have present about 0.5 mole to about 1.0 mole ofetherii'ying agent per mole of free alkali.

The alkali present in the reaction is usually sodiumhydroxide. However,any of the strong alkali hydroxides, including potassium hydroxide, aresuitable. It will be appreciated that, where the etherifying agent isadded in the form of free acid, alkali must be provided forneutralization of the acid above and beyond that required for bringingabout the etheriflcation reaction. The amount and concentration ofalkali with respect to water are factors governing the substitution ofthe final product. Any amount of alkali may be used in the process ofthe present invention as long as a product insoluble in the medium isobtained. Ordinarily, the ratio of alkali (as sodium hydroxide) tocellulose varies from about 0.1 part to about 2.0 parts per part ofcellulose. With respect to water, the ratio of alkali (as sodiumhydroxide) to water usually varies from about 0.02 to about 1.0 part ofsodium hydroxide per part of water. These ratios refer to free alkaliabove that consumed for neutralizing acid etherification agents.

The cellulose utilized in the reaction may be any reactive cellulose,such as chemically purified cotton linters, ramie fiber, or wood pulp.Cotton linters and 95% alpha-cellulose wood pulp are preferred rawmaterials.

The reaction may be carried out by merely mixing the ingredients in theetheriilcation reaction medium. Usually, the cellulose is firstsuspended in the secondary or tertiary alcohol reaction medium, asolution of alkali is added, and the etheriflcation agent is then addedas a solid or dissolved in water or the medium. The reaction mixture ismerely stirred until the desired reaction has been completed. Usually,the mixture is heated at some temperature between 25 and 100 C. tohasten the reaction. At temperatures of the order of 50-I5 C., areaction time of two six hours is usually sufllcient. It will be notedthat no special measures need be taken to prepare alkali cellulose. Mereaddition of sodium hydroxide to the suspension in the reaction mediumaccomplishes the desired result. However, if desired, alkali cellulosecan be prepared separately, as in many etherification reactions.

The product is recovered in a simple manner merely by draining ofireaction medium or removing it more completely by centrifuging and thenpurifying the product in a nonsolvent, such as methanol or ethanol,which will leach out the by-products. Purification is quite simple,since the product is substantially in the same form as the rawmaterial; 1. e., usually in an open fibrous condition. Thus, forexample, the fibrous reaction product is stirred in methanol or ethanol,free alkali is neutralized with acetic acid, and the mixture is thendrained oil and washed with additional methanol or ethanol. Usually, itis desirable to wash once with anhydrous methanol for the purpose ofdehydrating the product. Simple air drying provides a dry material readyfor shipment and use in a form which dissolves very readily.

What we claim and desire to protect by Letters Patent is:

1. In a process in which cellulose is reacted in the presence of strongalkali and water ,with a carboxyalkyl halide etheriiication agent toform a cellulose carboxyalkyl ether, the improvement which consists incarrying out the etheriflcation reaction in a reaction medium comprisingalcohol selected from the group consisting of isopropyl alcohol andtert.-butyl alcohol in an amount of at least 1.5 parts for each part ofwater present in the etheriiication reaction mixture and sumcient tomaintain the cellulose ether formed in a solid. undissolved statethroughout the reaction.

2. In a process in which cellulose is reacted in the presence or strongalkali and water with a carboxyalkyl halide etherification agent to forma cellulose carboxyalkyl ether, the improvement which consists incarrying out the etheriflcation reaction in isopropyl alcoholin anamount of at least 1.5 parts for each part of water present in theetheriiication reaction mixture and suflicient to maintain the celluloseether formed in a solid, undissolved state throughout the reaction.

3. In a process in which cellulose is reacted in the presence of strongalkali and water with a carboxyaikyl halide etheriflcation agent to forma cellulose carboxyalkyl ether, the improvement which consists incarrying out the etheriiication reaction in tert.-buty1 alcohol in anamount of at least 1.5 parts for each part of water present in theetheriiication reaction mixture and sums cient to maintain the celluloseether formed in a solid, undissolved state throughout thereaction.

4. A process for preparing a carboxyalkyl ether of cellulose whichcomprises agitating cellulose, i'ree alkali and water, and acarboxyalkyl etherifying agent in a quantity to etherify the cellulosein a reaction medium comprising alcohol selected from the groupconsisting of isopropyl alcohol and tert.-butyl alcohol in an amount ofat least 1.5 parts for each part of water present in the etherificationreaction mixture and sufficient to maintain the product formed in anundissolved state until the cellulose becomes etherifled, and recoveringthe resulting ether.

5. A process for the preparation of carboxymethylcellulose whichcomprises suspending cellulose in an alcohol selected from the oupconsisting of isopropyl alcohol and tert.-butyl alcohol in a quantitysuflicient to prevent solution of the cellulose ether formed in thereaction, adding free alkali and water and a chloracetic etheriiyingagent, said alcohol being present in an amount of at least 1.5 parts foreach part of water present in the etheriiication reaction mixture,heating the resulting mixture until the cellulose is etherified, andrecovering the resulting ether.

6. A process for the preparation of carboxymethylcellulose whichcomprises suspending cellulose in isopropyl alcohol in a quantitysuflicient to prevent solution of the cellulose ether formed in thereaction, adding free alkali and water and a chloracetic etherifyingagent, said alcohol be- 'ing present in an amount of at least 1.5 partsfor each part of water present in the etheriiication reaction mixture,heating the resulting mixture until the cellulose is etherifled, andrecovering the resulting ether.

."7. A process for the preparation of carboxy J methylcellulose whichcomprises suspending cellulose in tert.-buty'l alcohol in a quantitysuflicient to prevent solution of the cellulose ether formed in thereaction, adding tree alkali and water and a chloracetic etherifyingagent, said alcohol being present in an amount of at least 1.5 parts foreach part of water present in the etheriflcation reaction mixture,heating the resulting mixture until the cellulose is etheriiled, andrecovering the resulting ether.

8. A process for the preparation of cerboxylied, and recovering theresulting ether.

9. A process for the preparation of carbonymethylcellulose whichcomprises suspending cellulose in tert.-butyl alcohol in a quantitysumcient to prevent solution of the cellulose ether formed in thereaction, adding sodium hydroxide and water and sodium chloracetate,said alcohol being present in an amount of at least 1.5 parts for eachpart of water present in the etheriflcation reaction mixture, heatingthe resulting mixture at 25100 C. until the cellulose is etherifled, andrecovering the resulting ether.

EUGENE D. mJUG. JOHN S. TINBLEY.

REFERENCES CITlZD The following references are 01' record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,682,294 Lilienfeld Aug. 28,1928 2,110,526 Lorand Mar. 8, 1938 2,137,343 Maxwell Nov. 22, 19382,170,009 Clarke et a1 Aug. 22, 1939 2,236,523 Coolidge Apr. 1, 19412,236,545 Maxwell et a1 Apr. 1, 1941

1. IN A PROCESS IN WHICH CELLULOSE IS REACTED IN THE PRESENCE OF STRONGALKALI AND WATER WITH A CARBOXYALKYL HALIDE ETHERIFICATION AGENT TO FORMA CELLULOSE CARBOXYALIKYL ETHER, THE IMPROVEMENT WHICH CONSISTS INCARRYING OUT THE ETHERIFICATION REACTION IN A REACTION MEDIUM COMPRISINGALCOHOL SELECTED FROM THE GROUP CONSISTING OF ISOPROPYL ALCOHOL ANDTERT.-BUTYL ALCOHOL IN AN AMOUNT OF AT LEAST 1.5 PARTS FOR EACH PART OFWATER PRESENT IN THE ETHERIFICATION REACTION MIXTURE AND SUFFICIENT TOMAINTAIN THE CELLULOSE ETHER FORMED IN A SOLID, UNDISSOLVED STATETHROUGHOUT THE REACTION.